LONDON, United Kingdom —Natalie Massenet said she had planned to keep a low profile after she stepped down in 2015 as the executive chairwoman of Net-a-Porter, the popular luxury e-commerce site she founded.

After more than two-and-a-half years away, she is returning to e-commerce — but as a venture capitalist, not a fashion entrepreneur.

Ms. Massenet has teamed up with Nick Brown, an investor in consumer start-ups, to form Imaginary Ventures. The new firm announced on Tuesday that it has raised $75 million for its first fund, money that the two expect to pour into direct-to-consumer start-ups like Glossier, which sells beauty product to millennials, and Keeps, a hair-loss prevention company.

“I didn’t have a specific plan other than to take a break,” Ms. Massenet said. “But while talking to various entrepreneurs, I found that this was something I enjoyed.”

The two are focused on investing in e-commerce start-ups as they have become bigger and attracted takeover interest from existing retailers. The clothier Bonobos sold itself to Walmart for $310 million in June, for example, while the latest investment in mattress maker Casper valued it at $750 million.

Imaginary Ventures faces plenty of competition. Firms like Forerunner Ventures, Thrive Capital and Benchmark are active investors in e-commerce, on the hunt for the next Warby Parker (which recently raised $75 million) or Stitch Fix (which went public last year and is now valued at about $2 billion.)

Ms. Massenet and Mr. Brown are counting on Ms. Massenet’s Net-a-Porter experience and contacts to help Imaginary Ventures stand out. The firm has already invested in companies like Glossier and Everlane, an upstart clothing brand.

“Their understanding of consumers in today’s age and how to build great brands and guide entrepreneurs is unparalleled,” Michael Preysman, Everlane’s founder, said of the Imaginary founders in a statement.

Ms. Massenet began as a fashion journalist before founding Net-a-Porter in 2000. She left Net-a-Porter shortly after the e-commerce portal merged with rival Yoox in a deal with which she reportedly was unhappy.

Ms. Massenet had intended to largely retire, keeping a yellow Post-it note with the phrase “Just Say No” on her computer as a reminder of what to say to the flood of requests for business opportunities. Other than joining a Net-a-Porter rival, FarFetch, as co-chairwoman, she largely succeeded in doing just that.

Then came Mr. Brown. He was previously a partner at 14W Venture Partners, where he invested in the likes of Gwyneth Paltrow’s Goop lifestyle site and the shoe brand Allbirds.

“We had been looking at the market from different perspectives,” Ms. Massenet said. “I realized there was no better person in the space to work with than Nick.”

Over the course of 2016, the two decided to join forces and began talking to prospective investors. Backers of the new firm include Rick Caruso, a real estate developer based in Los Angeles, and two fellow fashion entrepreneurs, Tom and Ruth Chapman, who built and then sold Matchesfashion.com.

For Mr. Brown, the clearest sign that he had won over Ms. Massenet came when the Post-it note came off her computer.

“It was such an accomplishment to be able to yank it down,” he said.

By Michael J. de la Merced. This article originally appeared in The New York Times and was legally licensed through the NewsCred publisher network.